710 



ECOLOGY AND EVOLUTION 



When these several factors affecting the 

 degree of pathogenicity are evaluated, 

 there seem to be strong indications of an 

 evolution toward toleration between host 

 and parasite in long-estabHshed associa- 

 tions. In many instances the older associa- 

 tions tend to produce less pathological 

 effects in the host, probably because of 

 the mutual advantage of a decreased mor- 

 tality and the consequent selective differ- 

 ential. 



A number of investigators have doubted 

 or opposed the theory that natural selec- 

 tion influences the evolution of biotic 

 equiUbrium. Elton (1930) agrees that 

 natural selection is responsible for the evo- 

 lution of adaptations, but questions the 

 existence of any balance of nature or a re- 

 sultant regulation of numbers in a com- 

 munity. Elton's concept of balance seems 

 to be constancy, while we refer rather to 

 long-term relations of numbers that in- 

 clude many regular and irregular periodic- 

 ities as well as dispersal. Rather than con- 

 stancy, balance may be characterized by 

 tendencies to restore an equiUbrium during 

 periods of large or small fluctuations. Elton 

 shows many effects of numbers in one spe- 

 cies on numbers in other species, and such 

 effects often produce a long-term equiUb- 

 rium, as well as selection pressures guid- 

 ing evolution. Elton's use of the term "reg- 

 ulation" seems to be what we refer to as 

 "balance" or "equilibrium," and he readily 

 recognizes the fact that the "community 

 possesses some power of regulation." 



Nicholson (1933) recognized the exist- 

 ence of balance, but regards it as produced 

 and maintained by competition, not by 

 natural selection. It is true that balance 

 may be the result of opposing competitive 

 or offense-defense pressures without re- 

 ciprocal adaptation (Tinbergen, 1946). 

 However, the foregoing discussion seems to 

 show that an evolutionary development of 

 balance between populations of different 

 species has occurred in some cases at least. 

 To some degree it is similar to the dynamic 

 equiUbrium characteristic of individual or- 

 ganisms and intraspecies populations (pp. 

 436-441, 495-522). EquiUbrium in inter- 

 species systems often constitutes ecological 

 homeostasis. 



The evolution of toleration from exploi- 

 tation in the relations between two or more 

 associated species indicates a degree of re- 



ciprocal adjustment within an interspecie! 

 system. Let us now consider some of th« 

 more startUng cases of integration between 

 species found in mutualistic relations. 



MUTUALISM 



Mutually beneficial relationships between 

 species have often been classified as sym- 

 biosis (De Bary, 1879). This term, literally 

 meaning "Uving together," has been used 

 for mutuaUsm, parasitism, and commen- 

 saUsm (p. 243). The term mutualism 

 seems best when reciprocal beneficial re- 

 lationship is meant (pp. 245, 698). "Sym- 

 biosis" may be retained as a general terra 

 to include all aspects of physiologic and 

 ecologic species partnership. One of the 

 partners may be referred to as a symbiote 

 (also referred to as a symbiont; see Meyer, 

 1925; Steinhaus, 1946, p. 189). 



With a pronounced tendency of organ- 

 isms to evolve toward balanced equilibrium 

 with and toleration of other species within 

 the community, mutuaUsm between species 

 might evolve with a continuation of the 

 same evolutionary factors. Numerous in- 

 stances indicate that premutualistic ances- 

 tral types have been herbivores, predators, 

 parasites, or, on occasion, commensals, so 

 that exploitation and toleration preceded 

 mutualism in the majority of cases (Yonge, 

 1944; p. 253). Preadaptive mutuaUsm, a 

 phase of proto-cooperation, has been dis- 

 cussed elsewhere (pp. 395, 698). 



Subtle cooperation bordering on tolera- 

 tion may be expected to be rather common 

 (see pp. 398, 706), but more diflBcult to 

 detect than the more extreme cases of 

 mutualism. As an example, Crane's (1941) 

 observations (p. 620) on the courtship be- 

 havior of species of crabs (Uca) indicate 

 that closely related species within the same 

 habitat are more Ukely to have distinct 

 courtship behavior and color than are 

 closely related species geographically or 

 ecologically separated. There is a sugges- 

 tion in this instance that it is to the mutual 

 advantage of each species to prevent in- 

 efficient interspecific cross mating and to 

 foster species recognition within the same 

 area. We assume that selection pressure 

 producing divergent sexual behavior in 

 closely related species is strong within the 

 same association, and weak or absent if the 

 species are separated (Huxley, 1938, 



Examples of more obvious mutuaUsm 

 1938a). 



